The bug: Holding F3 + A and not letting go gives you the ability to clearly see nametags and particles through blocks like from players or lava particles for example.
Steps to reproduce:
Move players/mobs with nametags and lava where you can't see them
Hold F3 + A to x-ray through blocks to see players/mobs and lava (make sure you aren't too far away). If you computer freezes try lowering the render distance.
Expected behavior: You shouldn't be able to see anything through blocks, not even nametags so you can't see them easier due to the contrast from the sky/void.
Actual behavior: You are able to see particles through walls and nametags a lot easier as if there aren't any blocks in between.
Please attach some files. Better be a video.